Interface methods are not assignment compatible with method references or methods of object.

Boy I wish that QC was still up and QualityPortal was publicly indexable as that would have saved me quite a bit of time tracking this down. Luckily I got help from Stefan Glienke (who maintains the awesome Spring4D library based on modern Delphi compiler support) when I mentioned

How good are you with reference to function?
I’ve an odd compiler thing throwing errors when using interfaces but not with classes.

So, for posterity:

Unlike C#, in Delphi interface methods are not compatible with method references or methods of object.

This has many manifestations, which means you can get a variety of compiler errors. I’ve listed the ones I could find below, but presume there are more and if I find more will update this post.

These are the errors you can get:

E2010 Incompatible types: ‘T’ and ‘Procedure’

E2035 Not enough actual parameters

E2250 There is no overloaded version of ‘FirstOrDefault’ that can be called with these arguments

These are the (now defunct, but used to be publicly accessible) QC and QualityPortal (needs sign on) entries (thanks Stefan Glienke and Blaise Thorn for reporting these):